Geolocation of a mobile terminal in a CDMA communication system

ABSTRACT

A cellular radio system in which a base station receiver can receive, on the reverse link, data from a mobile terminal in one of four control modes. In the first mode, the mobile terminal sends an independent user pilot, not synchronized with the base station, on the reverse link and the user data channel is synchronized to this independent user pilot. In the second mode, the mobile terminal slaves its user pilot to the pilot it receives from the base station and the user data channel is synchronized with this slaved user pilot. This second mode allows the user terminal to receive round trip delay information for purposes of geolocation and rapid reacquisition. In the third mode, the mobile terminal slaves its user pilot to the incoming base station pilot, as in the case of mode two, but the user data channel operates in the orthogonal mode using the ranging information received from the base station. The phase relationship between the user pilot channel and the user data channel is calibrated. The user pilot carrier is also the carrier for the user data channel and can be used as the carrier reference for detecting the user data channel. In the fourth mode, the slaved pilot implementation of mode three is used for acquisition but, after acquisition, the user pilot code is phase shifted to be synchronous with the user data channel, thus also making it an orthogonal channel. In this mode, the pilots no longer contribute interference to the user data channels, within the cell, and can be transmitted at higher power levels.

CROSS REFERENCES TO RELATED APPLICATIONS

This patent is a continuation application of U.S. patent applicationSer. No. 09/280,327, filed Mar. 29, 1999, now U.S. Pat. No. 6,141,332,which is a continuation of U.S. patent application Ser. No. 08/807,228,filed Feb. 28, 1997, now U.S. Pat. No. 5,943,331.

BACKGROUND OF THE INVENTION

This invention relates to spread-spectrum communications and moreparticularly to a system and method using orthogonal codes and knowledgeof the distance between a mobile terminal and a base station to adjustand align the phase of an information channel to achieve orthogonalityat the base station.

DESCRIPTION OF THE RELEVANT ART

Referring to FIG. 1, message data, d(t), are processed byspread-spectrum modulator 51, using a message-chip-code signal, g₁(t),to generate a spread-spectrum data signal. The spread-spectrum datasignal is processed by transmitter 52 using a carrier signal at acarrier frequency f_(o), and transmitted over communications channel 53.

At a receiver, a spread-spectrum demodulator 54 despreads the receivedspread-spectrum signal, and the message data are recovered bysynchronous data demodulator 60 as received data. The synchronous datademodulator 60 uses a reference signal for synchronously demodulatingthe despread spread-spectrum signal. The square-law device 55, bandpassfilter 56 and frequency divider 57 are well known in the art forgenerating a reference signal from a received modulated data signal. ACostas Loop or other reference signal generating circuit is adequate forthis purpose.

In a fading channel, such as the ionosphere or any channel containingmultipath, or more generally, any channel in which the received signal'samplitude fluctuates with time, synchronous demodulation is notpractical since the phase of the incoming signal typically is not thesame as the phase of the reference. In such cases differential phaseshift keying (DPSK) is employed. With DPSK the received signal isdelayed by one symbol and multiplied by the underlayed signal. If theresulting phase is less than ±90° a 0-bit is declared, otherwise a 1-bitis declared. Such a system is complex and suffers degradation of about 6dB at error rates of 10⁻².

The prior art does not provide a system and method for synchronouslycommunicating, using spread-spectrum modulation, with a base station andin combination using range to the mobile terminal to achieveorthogonality at the base station.

OBJECTS OF THE INVENTION

A general object of the invention is a geolocation system and methodthat can be used as a personal communications service.

An object of the invention is a system and method for synchronouslycommunicating a modulated-data signal embedded in a CDMA signal, and forgeolocating a remote unit, which performs well whether or not the signalis fading.

Another object of the invention is a geolocation system and method whichuses a separate spread-spectrum channel as a pilot signal for a datalink for geolocating a remote unit and for demodulating a modulated-datasignal embedded in a CDMA signal.

An additional object of the invention is synchronousspread-spectrum-communications and geolocation system.

A further object of the invention is a spread spectrum system and methodfor using orthogonal codes and known range to a mobile terminal toachieve orthogonality of mobile terminal user data signals at the basestation.

A still further object of the invention is a system and method for usingorthogonal codes on a reverse link of a duplex radio channel.

The current cellular CDMA systems do not use orthogonal codes on thereverse link. In fact, the IS-95 systems use non-coherent detection onthe reverse link. This is because of the difficulty in synchronizing thespreading codes with each other as they arrive at the base station fromthe multiple mobile users. For codes to be orthogonal, the differentcodes have to start at essentially the same time and end at the righttime. Therefore, since the mobile user stations are at differentdistances from a base station, and probably moving, even if all thesignals are synchronized when they leave the mobile stations, thedifferent path lengths will make them non-synchronous when the signalsarrive at the base station.

There are at least three different signals that can gain in thedetection process if the sampling is done at the proper time or if thepredetermined waveforms are properly aligned in time. Both of theseconcepts, i.e., sampling at the proper time or having known waveformsaligned, are generally referred to as being synchronized. In the case ofcarrier synchronization, the correct carrier phase must be tracked. Thismeans the correct frequency is also followed and, therefore, a knownwaveform is phase aligned. In the case of PN code synchronization, it isnecessary to slip the phase of the locally generated PN code inreference to the received PN code until the two signals have exact phasealignment; this alignment is maintained by keeping the chip clock forthe locally generated PN code locked to the clock of the received PNcode. Again, this is phase aligning a known waveform.

In the case of the information signal, there has to be a degree ofuncertainty involved or there would be no information transmitted.Therefore, if the information is transmitted on a bit-by-by basis, adecision is made during each bit of information. If a noise averagingfilter or integrator is matched to the predetermined bit rate, notpredetermined phase of a predetermined waveform, and if the sample ismade at the end of the bit period such that the integration process hasreached a maximum, the phase or amplitude of the received signal can bemeasured to determine the information content. For instance a carriersine wave, a predetermined waveform at f_(c), continues over hundreds ofcycles at a predetermined phase. The information signal can then changethe phase to another predetermined and acceptable phase angle. Thischange in phase can represent a code which contains the information bit.The prior art contains a number of techniques for maintaining asynchronous local carrier even when the received carrier has its phaseoccasionally changed due to information.

In a CDMA system, there is a better way to derive a clean local carrier,at the receiver, than deriving it from the information channel. In aCDMA system it is possible to send the same RF carrier but with adifferent PN code superimposed on it. This signal has no unknowninformation on it; it is a completely predetermined signal known on bothends of the link. Since this signal has a different code than the userinformation channel code, it is completely resolvable from the userinformation channel. Therefore, the two signals can occupy the samespectrum at the same time and only cause minor interference to eachother. This signal is called a pilot channel and can be filtered with anarrow filter at the receiver which allows it to be a very stablereference. The user information channel phase is then compared to thisclean reference to determine what changes are made to reflect theinformation on the user information channel. On the forward link, thesame pilot channel is used as the reference for many mobile userstations. As a result, the power of the pilot channel can be madeseveral times greater than the power of an individual user informationchannel and still have a small impact on the total power transmitted bythe base station. This power factor, combined with the fact that all thesignals have the same originating point and the same timing source,makes it easy to use orthogonal codes on this forward transmission link.All the mobile users receive the same composite CDMA forwardtransmission signal and use the same pilot channel to extract theirassigned user information channel from the composite CDMA signal.

The complexity in deriving and detecting orthogonal codes results inpractical orthogonal codes being relatively short, i.e., 64 chips forIS-95 systems, with some other proposals at 128 chips. These short codeslimit the available pre-detection processing gain. Since the codes arecontinuously repeated, the resulting structure of the spectrum consistsof a small number of lines with large spaces between the lines; this isnot very noise-like which is the result that is desired. Therefore, asin the case of IS-95, a longer, more noise-like code is superimposed ontop of the orthogonal codes. If the pilot channel code is also one ofthe orthogonal codes, it will not contribute noise into the informationchannels. In the case of the IS-95, the pilot is Walsh code 0 whichmeans it is just the superimposed noise-like code, because the Walsh 0code is all zeros. To achieve the full cancellation of companionorthogonal codes, the codes must be perfectly aligned with all the zerocrossings happening at exactly the same time. Any misalignment createsunmatched glitches that will cause interference to the desired signal.On the forward link, the multiple signals transmitted to all the mobilesare added together to form one composite CDMA signal. As a result, thesignals are in perfect alignment with each other and since all signalstravel the same path, they will stay aligned. Therefore, orthogonalcodes are practical and straight-forward to implement. The onlydisadvantages are limited processing gain and the limited number ofavailable codes.

Utilizing orthogonal codes on the reverse link is more difficult sincethe different codes are originating from the different mobile stationsthat are randomly distributed as a function of distance from the basestation where the signals must arrive in perfect alignment. This meansthat, to have all the signals arrive in synchronism at the base station,each mobile station would have to start its reference point at adifferent time to compensate for the variance in path length. This hasbeen considered too difficult to be practical in current systems. U.S.Pat. No. 5,404,376 addresses this issue by having the base stationestablish and broadcast a relationship between the mobile received C/Iand distance that is continually updated on the basis of measured data.Based on this relationship, the mobile estimates the PN phase that willmake the PN code arrive at the base station approximately in sync withother mobile transmissions. There are many problems with this approach.In particular, it is difficult to maintain a consistent relationshipbetween C/I and distance from the base station. Even at best, thisrelationship will depend on the direction taken by the propagation path.U.S. Pat. No. 5,404,376 proposes some complicated techniques wherebycorrection factors are added to accommodate for the direction, orsector, wherein the mobile is located. At best the result is only anestimate and there is still a large uncertainty that has to be searched.This invention overcomes these difficulties by addressing thedetermination of the distance of the mobile from the base station in aunique, simple and direct manner.

SUMMARY OF THE INVENTION

According to the present invention, as embodied and broadly describedherein, a spread spectrum code division multiple access (CDMA)communications system and method for communicating over a duplex radiochannel is provided comprising at least one base station and a pluralityof mobile terminals. Message data are communicated between the basestations and the mobile terminals. Message data includes, but are notlimited to, digitized voice, computer data, facsimile data, video data,etc. The base station communicates base-message data over a forwardchannel to the plurality of mobile terminals. A mobile terminalcommunicates remote-message data over a reverse link to the basestation. Base-message data are defined herein to be message dataoriginating from a base station, and remote-message data are definedherein to be message data originating from a mobile terminal.

Remote message data is spread spectrum processed using a pseudo-noisecode to generate spread-spectrum-processed-remote-message data. Aremote-pilot signal is combined with thespread-spectrum-processed-remote-message data to generate a remote-CDMAsignal. The remote-CDMA signal contains the remote-pilot signal and adata signal.

The remote-CDMA signal is transmitted from the mobile terminal to thebase station on a reverse channel of the duplex radio channel. The basestation receives the remote-CDMA signal and splits the remote-CDMAsignal into a pilot channel and a data channel. The base stationgenerates a base-pilot signal and a base-pilot-reference signal. Thebase-pilot-reference signal is split and delayed to generate an on-timeversion of the base-pilot-reference signal, an early version of thebase-pilot-reference signal, and a late version of thebase-pilot-reference signal. The on-time, the early and the lateversions of the base-pilot-reference signal are used to correlate out anon-time, an early, and a late version, respectively, of the remote-pilotsignal. The base station also generates a base-data-reference signal andcorrelates out the data signal using the base-data reference signal.

The phase of the remote-pilot signal is tracked and, in response to apeak in the remote-pilot signal, an acquisition signal is outputsignifying synchronization of the remote-pilot signal and thebase-pilot-reference signal. In response to the acquisition signal, thecode phase difference between the base-pilot signal and thebase-pilot-reference signal is measured to determine the range betweenthe mobile terminal and the base station. The range is transmitted tothe mobile terminal on a forward channel and, in response to the range,the mobile terminal adjusts the phase of the pseudo-noise code to adjustan arrival time of the data signal at the base station and to achieveorthogonality with other arriving mobile terminal data signals at thebase station.

The base station can receive, on the reverse link of the duplex channel,data from the mobile terminal in one of four control modes. In the firstmode, the mobile terminal sends an independent user pilot, notsynchronized with the base station pilot, on the reverse link and theuser data channel is synchronized to this independent user pilot. In thesecond mode, the mobile terminal slaves its user pilot to the pilot itreceives from the base station and the user data channel is synchronizedwith this slaved user pilot. This second mode allows the user terminalto receive round trip delay information for purposes of geolocation andrapid reacquisition. In the third mode, the mobile terminal slaves itspilot to the incoming base station pilot, as in the case of mode two,but the user data channel operates in the orthogonal mode using theranging information received from the base station. The phaserelationship between the user pilot channel and the user data channel iscalibrated. The user pilot carrier is also the carrier for the user datachannel and can be used as the carrier reference for detecting the userdata channel. In the fourth mode, the slaved pilot implementation ofmode three is used for acquisition but, after acquisition, the userpilot code is phase shifted to be synchronous with the user datachannel, thus also making it an orthogonal channel. In this mode, thepilots no longer contribute interference to the user data channels,within the cell, and can be transmitted at higher power levels.

Additional objects and advantages of the invention are set forth in partin the description which follows, and in part are obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention also may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a prior art scheme for synchronously recovering message data;

FIG. 2 shows a synchronous spread-spectrum system with a bitsynchronizer, synchronized to a generic chip code generator according tothe present invention;

FIG. 3A shows a synchronous spread spectrum transmitter system for aplurality of message data;

FIG. 3B shows a spread spectrum receiver using a synchronous detectorfor receiving a plurality of spread-spectrum processed signals;

FIG. 3C shows a spread spectrum receiver using a nonsynchronous detectorfor receiving a plurality of spread-spectrum processed signals;

FIG. 4 shows a synchronous spread-spectrum demodulating method;

FIG. 5 is a block diagram of a base station for communicatingsynchronously with, and geolocating a remote unit;

FIG. 6 is a block diagram of a remote unit for communicating with a basestation and geolocation;

FIGS. 7 a and 7 b are block diagrams of a mobile terminal in accordancewith the orthogonal code synchronization system and method of thepresent invention; and

FIGS. 8 a and 8 b are block diagrams of a base station of the orthogonalcode synchronization system and method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals indicate like elementsthroughout the several views.

The spread-spectrum communications and orthogonal code synchronizationsystem and method of the present invention is an extension of aninvention disclosed in a U.S. patent application entitled,SYNCHRONOUS-SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD, by DonaldL. Schilling, having Ser. No. 07/626,109 and filing date of Dec. 14,1990, now issued U.S. Pat. No. 5,228,056. For completeness ofdisclosure, the following discussion includes the disclosure presentedin the original patent application, and subsequently goes into adiscussion of orthogonal code synchronization according to the presentinvention.

The spread spectrum signals of the present invention are designed to be“transparent” to other users, i.e., spread spectrum signals are designedto provide negligible interference to the communication of otherexisting users. The presence of a spread spectrum signal is difficult todetermine. This characteristic is known as low probability ofinterception (LPI) and low probability of detection (LPD). The LPI andLPD features of spread spectrum allow transmission between users of aspread spectrum CDMA communications system without the existing users ofthe mobile cellular system experiencing significant interference. Thepresent invention makes use of LPI and LPD with respect to predeterminedchannels in the mobile cellular system or in the fixed-service microwavesystem. By having the power level of each spread spectrum signal belowthe predetermined level, then the total power from all spread spectrumused within a cell does not interfere with mobile users in a mobilecellular system, or with microwave users in the fixed-service microwavesystem.

Spread spectrum is also “jam” or interference resistant. A spreadspectrum receiver spreads the spectrum of the interfering signal. Thisreduces the interference from the interfering signal so that it does notnoticeably degrade performance of the spread spectrum system. Thisfeature of interference reduction makes spread spectrum useful forcommercial communications, i.e., the spread spectrum waveforms can beoverlaid on top of existing narrowband signals.

The present invention employs direct sequence spread spectrum, whichuses a phase modulation technique. Direct sequence spread spectrum takesthe power that is to be transmitted and spreads it over a very widebandwidth so that the power per unit bandwidth (watts/hertz) isminimized. When this is accomplished, the transmitted spread spectrumpower received by a mobile cellular user or a microwave user, having arelatively narrow bandwidth, is only a small fraction of the actualtransmitted power.

In a fixed-service microwave system, by way of example, if a spreadspectrum signal having a power of 10 milliwatts is spread over afixed-service microwave bandwidth of 10 MHz and a microwave user employsa communication system having a channel bandwidth of only 2 MHz, thenthe effective interfering power due to one spread spectrum signal, inthe narrow band communication system, is reduced by the factor of 10MHz/2 MHz. For fifty concurrent users of spread spectrum, the power ofthe interfering signal due to spread spectrum is increased by fifty.

The feature of spread spectrum that results in interference reduction isthat the spread spectrum receiver actually spreads the received energyof any interferer over the same wide bandwidth, 10 MHz in the presentexample, while compressing the bandwidth of the desired received signalto its original bandwidth. For example, if the original bandwidth of thedesired message data is only 30 kHz, then the power of the interferingsignal produced at a base station is reduced by 10 MHz/30 kHz.

Direct sequence spread spectrum achieves a spreading of the spectrum bymodulating the original signal with a very wideband signal relative tothe data bandwidth. This wideband signal is chosen to have two possibleamplitudes, +1 and −1, and these amplitudes are switched, in a“pseudo-random” manner, periodically. Thus, at each equally spaced timeinterval, a decision is made as to whether the wideband modulatingsignal should be +1 or −1. If a coin were tossed to make such adecision, the resulting sequence would be truly random. However, in sucha case, the receiver would not know the sequence a-priori and could notproperly receive the transmission. Instead, a chip-code generatorgenerates electronically an approximately random sequence, called apseudo-random sequence, which is known a-priori to the transmitter andreceiver.

Code Division Multiple Access

Code division multiple access (CDMA) is a direct sequence spreadspectrum system in which a number, at least two, of spread-spectrumsignals communicate simultaneously, each operating over the samefrequency band. In a CDMA system, each user is given a distinct chipcode. This chip code identifies the user. For example, if a first userhas a first chip code, g₁(t), and a second user has a second chip code,g₂(t), etc., then a receiver, desiring to listen to the first user,receives at its antenna all of the energy sent by all of the users.However, after despreading the first user's signal, the receiver outputsall the energy of the first user but only a small fraction of theenergies sent by the second, third, etc., users.

CDMA is interference limited. That is, the number of users that can usethe same spectrum and still have acceptable performance is determined bythe total interference power that all of the users, taken as a whole,generate in the receiver. Unless one takes great care in power control,those CDMA transmitters which are close to the receiver causeoverwhelming interference. This effect is known as the “near-far”problem. In a mobile environment the near-far problem could be thedominant effect. Controlling the power of each individual mobile remoteuser is possible so that the received power from each mobile remote useris the same. This technique is called “adaptive power control”. See U.S.Pat. No. 5,093,840, having issue date of Mar. 3, 1992, entitled,ADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUM SYSTEM AND METHOD, byDonald L. Schilling, which is incorporated herein by reference.

The spread spectrum communications system of the present invention is acode division multiple access (CDMA) system. Spread spectrum CDMA cansignificantly increase the use of spectrum. With CDMA, each user in acell uses the same frequency band. However, each CDMA signal has aseparate pseudo random code which enables a receiver to distinguish adesired signal from the remaining signals. Remote users in adjacentcells use the same frequency band and the same bandwidth, and therefore“interfere” with one another. A received signal may appear somewhatnoisier as the number of users' signals received by a PCN base stationincreases.

Each unwanted user's signal generates some interfering power whosemagnitude depends on the processing gain. Remote users in adjacent cellsincrease the expected interfering energy compared to remote users withina particular cell by about 50%, assuming that the remote users areuniformly distributed throughout the adjacent cells. Since theinterference increase factor is not severe, frequency reuse is notemployed.

Each spread spectrum cell can use a full 10 MHz band for transmissionand a full 10 MHz band for reception. Hence, using a chip rate of fivemillion chips per second and a coding data rate of 4800 bps results inapproximately a processing gain of 1000 chips per bit. It is well knownto those skilled in the art that the maximum number of CDMA remote usersthat can concurrently use a frequency band is approximately equal to theprocessing gain.

Orthogonal Codes

A pilot on the return link is now considered to be practical, because itdecreases the C/I that is required to achieve the desired E_(b)/N_(o),as disclosed in U.S. Pat. No. 5,506,864 and U.S. Pat. No. 5,544,156.This improvement derives from the ability to use synchronous or coherentdetection. As described in these patents, the use of a pilot or genericchip-code improves the performance of both orthogonal and non-orthogonalcoded links. Since, for orthogonal channels, each mobile requires uniquepilot and information codes, the number of active users is reduced bytwo. If there are a limited number of codes this could have a seriousimpact. U.S. Pat. No. 5,506,864 uses the pilot from the mobile tomeasure the distance between the base station and the mobile usingnon-orthogonal codes. This invention expands upon that patent to includeorthogonal codes and uses the knowledge of the distance to the mobileterminal to adjust the phase of the information channel to make it alignwith the other mobile signals arriving at the base station. The mobilereceives the pilot or generic chip code signal from the base station anduses the timing and phase of the base station pilot signal to originatethe remote-pilot signal it sends to the base station. That is, thereturned pilot has no delay going through the mobile; the returned pilotlooks like a radar reflection off the mobile. It is, of course, strongerin signal strength and, because there are many remote pilots that willbe returned to the base station, it is a different but similarpseudo-noise code than the base station pilot pseudo-noise code.

The base station receives the pilot signals from all active mobiles andmeasures the phase difference, when possible down to 0.1 chip, betweenthe returned pseudo-noise sequence and the transmitted pseudo-noisesequence for each mobile station. What is measured is the round tripdelay; the actual distance is one half this number measured in chips, to0.1 chip. This information is transmitted to the mobile user and, if themobile user is operating in an orthogonal mode on the return link, themobile user uses this information to adjust the phase of the PN code onthe remote message to arrive at the base station at a is predeterminedtime, as established by the base station. Therefore, the PN code of theremote pilot and the remote user message channels are at differentphases, but they both have the same carrier signal and the pilot carriercan be used to generate a reference for coherent detection in the usermessage channel.

The data sampling point is usually tied to the repetition rate of the PNsequence and will be adjusted in phase to comply with the data timing onthe user message channel. Therefore, it is possible to significantlyreduce the mutual interference caused by the user message channels thatare in communication with a common base station.

The interference from the mobiles in adjacent cells is not orthogonaland appears as non-orthogonal interference. Most orthogonal code CDMAsystems utilize sectored antennas to obtain code reuse and reduce theinterference. Therefore, at the edge of the cell, across the face of thesector, the mobiles in each cell transmit at maximum power and causeradiation into both cells at the maximum energy. However, as the mobileusers in the adjacent cell move toward their base station, they reducetheir power to keep it the same as when they were on the edge of thecell. Assuming a fourth power attenuation curve, they reduce their powerat a fourth power vs. distance rate and since they are also moving awayfrom the base station being interfered with, their reduced transmittedpower level, reduced as the fourth power, travels a further distancewhich is also decreased at the fourth power factor. This doubles theeffect of the fourth power factor which means the adjacent cellinterference from mobile users is much less than if power control werenot used. Therefore, the external interference, i.e., interference frommobile users operating with other base stations, introduced at theprimary base station is down at least 6 db from the interference causedwithin the cell from other mobile users operating with the primary basestation. Therefore, it is possible to increase the number of users by afactor of four. As stated previously, each active mobile user transmitsa pilot channel and an information or message channel. The informationchannels are adjusted so that they are orthogonal when they arrive atthe base station. The pilot channels, however, are not orthogonal but,after the information channel is functioning, the pilot channel power isreduced by 6 db. Therefore, even with the external interference and thepilot channels, the capacity is doubled as the result of the presentinvention.

Still another improvement is possible by shifting the phase of theremote pilot after acquisition to coincide with the user informationchannel. When this is accomplished the remote pilots also becomeorthogonal and the only interference is the external interference thatis radiated into the prime cell from users in adjacent cells. As statedpreviously, this interference is down at least 6 db, resulting in afourfold increase in capacity. The code tracking on the reverse linkbecomes more difficult since the error is generated in the base stationand the oscillator that is controlled by this error voltage is in themobile station. Therefore, the forward link has to be used to transmitthis error voltage to the mobile station. Generally, the range changesrelatively slowly and this remote control of the mobile code clock isnot a problem. When sudden fluctuations occur that are significantenough to cause rapid severe misalignment, the mobile shifts the remotepilot code back to the acquisition mode. Upon reacquisition andcompletion of the necessary adjustments to bring the information channelback into proper alignment, the mobile switches back to the orthogonaltracking mode. Therefore the non-orthogonal remote pilots are only “on”a small portion of the time and the resulting impact on capacity issmall. Capacity should still be close to four times that of anon-orthogonal code system if there are enough orthogonal codes in thecode set to actually capitalize on this advantage.

Synchronous Spread Spectrum Communications

As illustratively shown in FIG. 2, a spread spectrum code divisionmultiple access (CDMA) communications system for use over acommunications channel 110 is provided comprising generic means, messagemeans, spreading means, summer means, transmitting means,generic-spread-spectrum-processing means,message-spread-spectrum-processing means, acquisition and trackingmeans, detection means and synchronous means. The generic means andmessage means are embodied as a transmitter-generic-chip-code generator101 and transmitter-message-chip-code generator 102. The spreading meansis shown as an EXCLUSIVE-OR device 103, which may be an EXCLUSIVE-ORgate. Summer means is a combiner 105 and the transmitting means includesa transmitter which is embodied as a signal source 108 coupled tomodulator 107. The transmitter-message-chip-code generator 102 iscoupled to the EXCLUSIVE-OR device 103. Thetransmitter-generic-chip-code generator 101 is shown coupled to thetransmitter-message-chip-code generator 102 and the source for messagedata. The EXCLUSIVE-OR device 103 and the transmitter-generic-chip-codegenerator 101 are coupled to the combiner 105. The modulator 107 iscoupled between the combiner 105 and the communications channel 110.

At the receiver the generic-spread-spectrum-processing means is embodiedas the receiver-generic-chip-code generator 121, the generic mixer 123and the generic-bandpass filter 125. The generic mixer 123 is coupledbetween the receiver-generic-chip-code generator 121 and thegeneric-bandpass filter 125. The message-spread-spectrum-processingmeans is embodied as a receiver-message-chip-code generator 122, amessage mixer 124 and a message-bandpass filter 126. The message mixer124 is coupled between the receiver-message-chip-code generator 122 andthe message-bandpass filter 126. A power splitter 115 is coupled betweenthe communications channel 110, and the generic mixer 123 and themessage mixer 124.

The acquisition and tracking means is embodied as an acquisition andtracking circuit 131. The acquisition and tracking circuit 131 iscoupled to an output of the generic-bandpass filter 125, and to thereceiver-generic-chip-code generator 121. The receiver-message-chip-codegenerator 122 preferably is coupled to the receiver-generic-chip-codegenerator 121.

The detection means is embodied as a detector 139. The detector 139 iscoupled to the message-bandpass filter 126 and the generic-bandpassfilter 125. The detector 139 may be a nonsynchronous detector such as anenvelope detector or square-law detector. Alternatively, the detector139 may be a synchronous detector, which uses a recovered-carrier signalfrom the generic-bandpass filter 125.

The synchronous means includes bit means, a lowpass filter 128 andelectronic switch 130. The bit means is embodied as a bit synchronizer129. The lowpass filter 128 and electronic switch 130 are coupled to thebit synchronizer 129. The bit synchronizer 129, as shown in FIG. 2,preferably is coupled to the receiver-generic-chip-code generator 121.Alternatively, the bit synchronizer 129 may be coupled to an output ofthe detector 139.

The transmitter-generic-chip-code generator 101 generates ageneric-chip-code signal, g₀(t), and the transmitter-message-chip-codegenerator 102 generates a message-chip-code signal, g₁(t). Synchronoustiming of the message data, d₁(t), and the message-chip-code signal, inFIG. 2, is provided by the generic-chip-code signal, although othersources can be used such as a common clock signal for synchronization.The EXCLUSIVE-OR device 103 generates a spread-spectrum signal byspread-spectrum processing message data with the message-chip-codesignal. The spread-spectrum processing may be accomplished by modulo-2adding the message data to the message-chip-code signal. The combiner105 combines the generic-chip-code signal with thespread-spectrum-processed signal. The combined generic-chip-code signaland spread-spectrum-processed signal may be a multilevel signal, havingthe instantaneous voltage levels of the generic-chip-code signal and thespread-spectrum-processed signal.

The modulator 107, as part of the transmitter, modulates the combinedgeneric-chip-code signal and spread-spectrum-processed signal by acarrier signal, cos ω_(o)t, at a carrier frequency, f_(o). The modulatedgeneric-chip-code signal and spread-spectrum processed signal aretransmitted over the communications channel 110 as a code divisionmultiple access (CDMA) signal, x_(c)(t). Thus, the CDMA signal includesthe generic-chip-code signal and the spread-spectrum-processed signal asif they were each modulated separately, and synchronously, on separatecarrier signals having the same carrier frequency, f_(o), andtransmitted over the communications channel.

At a receiver, the generic-spread-spectrum-processing means recovers thecarrier signal, cos ω_(o)t, from the CDMA signal, x_(c)(t), and themessage-spread-spectrum-processing means despreads the CDMA signal,x_(c)(t), as a modulated-data signal, d₁(t). More particularly,referring to FIG. 2, the CDMA signal received from the communicationschannel 110, is divided by power splitter 115. Thereceiver-generic-chip-code generator 121 generates a replica of thegeneric-chip-code signal, g₀(t). The generic mixer 123 uses the replicaof the generic-chip-code signal for despreading the CDMA signal,x_(c)(t), from the power splitter 115, as a recovered-carrier signal.The spread-spectrum channel, of the CDMA signal having thegeneric-chip-code signal, g₀(t) cos ω_(o)t, generally does not includedata so that despreading the CDMA signal produces the carrier signal,only. The generic-bandpass filter 125 filters the recovered-carriersignal at the carrier frequency, or equivalently, at an intermediatefrequency. In comparison to the message-bandpass filter 126 which has abandwidth sufficiently wide for filtering a modulated-data signal, thegeneric-bandpass filter 125 can have a very narrow bandwidth forfiltering the recovered-carrier signal. The very narrow bandwidth of thegeneric-bandpass filter 125 assists in extracting the recovered-carriersignal from noise.

The acquisition and tracking circuit 131 acquires and tracks therecovered-carrier signal from an output of the generic-bandpass filter125. The replica of the generic-chip-code signal from thereceiver-generic-chip-code generator 121 is synchronized to therecovered-carrier signal via acquisition and tracking circuit 131.

The receiver-message-chip-code generator 122 generates a replica of themessage-chip-code signal, g₁(t). The replica of the message-chip-codesignal, g₁(t), is synchronized to the replica of the generic-chip-codesignal, g₀(t), from the receiver-generic-chip-code generator 121. Thus,the receiver-message-chip-code generator 122, via synchronization to thereceiver-generic-chip-code generator 121, has the same synchronizationas the transmitter-message-chip-code generator 102 via synchronizationto the transmitter-generic-chip-code generator 101. Accordingly, thespread-spectrum communications channel having the generic-chip-codesignal provides coherent spread-spectrum demodulation of thespread-spectrum channels with data.

The message mixer 124 uses the replica of the message-chip-code signalfor despreading the CDMA signal from the power splitter 115, to generatea modulated-data signal, d₁(t) cos ω_(o)t. The modulated-data signaleffectively is the message data modulated by the carrier signal. Themessage-bandpass filter 126 filters the modulated-data signal at thecarrier frequency, or equivalently at an intermediate frequency (IF).Down converters, which convert the modulated-data signal to an IF,optionally may be used without altering the cooperative functions orteachings of the present invention.

The detector 139 demodulates the modulated-data signal as a detectedsignal. The detected signal is filtered through lowpass filter 128,sampled by electronic switch 130 and outputted as received data, d₁(t).The received data, without errors, are identical to the message data.The lowpass filter 128 and electronic switch 130 operate in an“integrate and dump” function, respectively, under the control of thebit synchronizer 129.

The bit synchronizer 129 controls the integrating and dumping of lowpassfilter 128 and electronic switch 130. The bit synchronizer 129preferably derives synchronization using the replica of thegeneric-chip-code signal from the receiver-generic-chip-code generator121 as illustrated in FIG. 2. The bit synchronizer 129 also may derivesynchronization from an output of the detector 139, as illustrated inFIG. 1.

In a preferred embodiment, the bit synchronizer 129 receives the replicaof the generic-chip-code signal, g₀(t), from thereceiver-generic-chip-code generator 121. The replica of thegeneric-chip-code signal, by way of example, may include a chip codewordhaving 8250 chips. Assuming that there are eleven bits per chipcodeword, then there are 750 chips per bit of data. Since the replica ofthe generic-chip-code signal provides information to the bitsynchronizer 129 as to where the chip codeword begins, the bitsynchronizer 129 thereby knows the timing of the corresponding bits forsynchronization.

The present invention further may include transmitting as the CDMAsignal, a plurality of spread-spectrum-processed signals for handling aplurality of message data. In this case the invention includes aplurality of message means and a plurality of spreading means. Referringto FIG. 3A, the plurality of message means may be embodied as aplurality of transmitter-message-chip-code generators and the pluralityof spreading means may be embodied as a plurality of EXCLUSIVE-OR gates.The plurality of transmitter-message-chip-code generators generates aplurality of message-chip-code signals. In FIG. 3A, the plurality oftransmitter-message-chip-code generators is shown as firsttransmitter-message-chip-code generator 102 generating firstmessage-chip-code signal, g₁(t), second transmitter-message-chip-codegenerator 172 generating second message-chip-code signal, g₂(t), throughN^(th) transmitter-message-chip-code generator 182 generating N^(th)message-chip-code signal, g_(N)(t). The plurality of EXCLUSIVE-OR gatesis shown as first EXCLUSIVE-OR gate 103, second EXCLUSIVE-OR gate 173,through N^(th) EXCLUSIVE-OR gate 183. The plurality of EXCLUSIVE-ORgates generates a plurality of spread-spectrum-processed signals bymodulo-2 adding the plurality of message data d₁(t), d₂(t), . . . ,d_(N)(t) with the plurality of message-chip-code signals g₁(t), g₂(t), .. . , g_(N)(t), respectively. More particularly, the first message data,d₁(t), are modulo-2 added with the first message-chip-code signal,g₁(t), the second message data, d₂(t), are modulo-2 added with thesecond message-chip-code signal, g₂(t), through the N^(th) message data,d_(N)(t), which are modulo-2 added with the N^(th) message-chip-codesignal, g_(N)(t).

The transmitter-generic-chip-code generator 101 is coupled to theplurality of transmitter-message-chip-code generators and the source forthe plurality of message data, d₁(t), d₂(t), . . . d_(N)(t). Thegeneric-chip-code signal g₀(t), in a preferred embodiment, providessynchronous timing for the plurality of message-chip-code signals g₁(t),g₂(t), . . . , g_(N)(t), and the plurality of message data d₁(t), d₂(t),. . . , d_(N)(t).

The combiner 105 combines the generic-chip-code signal and the pluralityof spread-spectrum-processed signals, by linearly adding thegeneric-chip-code signal with the plurality of spread-spectrum-processedsignals. The combined signal typically is a multilevel signal, which hasthe instantaneous voltage levels of the generic-chip-code signal and theplurality of spread-spectrum-processed signals.

The modulator 107, as part of the transmitter, modulates the combinedgeneric-chip-code signal and the plurality of spread-spectrum-processedsignals by a carrier signal, cos ω_(o)t, at a carrier frequency, f_(o).The modulated generic-chip-code signal and the plurality ofspread-spectrum processed signals are transmitted over thecommunications channel 110 as a CDMA signal, x_(c)(t). The CDMA signal,x_(c)(t) has the form:${x_{c}\quad(t)} = {{g_{0}\quad(t)} + {\sum\limits_{1}^{N}\quad{\left\lbrack {{g_{i}\quad(t)} + {d_{i}\quad(t)}} \right\rbrack\cos\quad\omega_{0}\quad t}}}$Thus, the CDMA signal includes the generic-chip-code signal and theplurality of spread-spectrum-processed signals as if they were eachmodulated separately, and synchronously, on separate carrier signalswith the same carrier frequency, f_(o), and transmitted over thecommunications channel.

The present invention includes receiving a CDMA signal which has aplurality of spread-spectrum-processed signals. The receiver furtherincludes a plurality of message-spread-spectrum processing means, aplurality of detection means and a plurality of synchronous means. Theplurality of message-spread-spectrum-processing means, as shown in FIG.3B, may be embodied as a plurality of message-chip-code generators, aplurality of message mixers and a plurality of message-bandpass filters.A mixer is connected between a respective message-chip-code generatorand message-bandpass filter. The plurality of message mixers is coupledto the power splitter 115. More particularly, the plurality ofmessage-chip-code generators is shown embodied as firstmessage-chip-code generator 122, second message-chip-code generator 172,through N^(th) message-chip-code generator 182. The plurality of messagemixers is shown as first message mixer 124, second message mixer 174through N^(th) message mixer 184. The plurality of message-bandpassfilters is shown as first message-bandpass filter 126, secondmessage-bandpass filter 176, through N^(th) message-bandpass filter 186.

The plurality of detection means may be embodied as a plurality ofsynchronous detectors which is shown as first synchronous detector 127,second synchronous detector 177 through N^(th) synchronous detector 187.Each of the plurality of synchronous detectors are coupled to one of theplurality message-bandpass filters.

The plurality of synchronous means may include a bit synchronizer 129, aplurality of lowpass filters and a plurality of electronic switches. Theplurality of lowpass filters is shown as first lowpass filter 128,second lowpass filter 178, through N^(th) lowpass filter 188. Theplurality of electronic switches is shown as first electronic switch130, second electronic switch 180 through N^(th) electronic switch 190.Each of the plurality of synchronous detectors is coupled to an outputof the generic-bandpass filter 125. The recovered-carrier signal fromthe generic-bandpass filter 125 serves as the reference signal forsynchronously demodulating each of the plurality of message-data signalsby the plurality of synchronous detectors, as a plurality of receiveddata, d₁(t), d₂(t), . . . , d_(N)(t).

The detection means alternatively may be embodied as a plurality ofnonsynchronous detectors, such as envelope detectors 139, 189, 199, asshown in FIG. 3C. Typically, the nonsynchronous detectors do not requirethe recovered-carrier signal.

The bit synchronizer 129 derives timing from the replica of thegeneric-chip-code signal, g₀(t), and controls the timing of theintegrating and dumping functions of the plurality lowpass filters andthe plurality of electronic switches.

With the use of the invention as embodied in FIG. 3B, ageneric-spread-spectrum channel, as part of the CDMA signal, providesthe recovered-carrier signal, as discussed previously. The acquisitionand tracking circuit 131 acquires and tracks the recovered-carriersignal from an output of the generic-bandpass filter 125. The replica ofthe generic-chip-code signal from the receiver-generic-chip-codegenerator 121 is synchronized to the recovered-carrier signal viaacquisition and tracking circuit 131. The receiver-generic-chip-codegenerator 121 generates a replica of the generic-chip-code signal,g₀(t), which provides timing to bit synchronizer 129 and to theplurality of receiver-message-chip-code generators 122, 172, 182.

The present invention also includes a method for synchronouslydemodulating a CDMA signal. Message data are input to the spreadingmeans. Referring to FIG. 4, the method comprises the steps of generating403 a generic-chip-code signal. The method further includes generating405 message data synchronized to the generic-chip-code signal, andgenerating 407 a message-chip-code signal synchronized to thegeneric-chip-code signal. Message data are processed, using aspread-spectrum modulator, with the message-chip-code signal to generatea spread-spectrum-processed signal. The generic-chip-code signal iscombined 409 with the spread-spectrum-processed signal. The methodtransmits 411 the combined generic-chip-code signal andspread-spectrum-processed signal on a carrier signal over thecommunications channel as a CDMA signal.

At a receiver, the method includes recovering 413 the carrier signalfrom the CDMA signal and despreading 415 the CDMA signal as amodulated-data signal. The recovered-carrier signal is used tosynchronize the step of despreading the CDMA signal and to optionallysynchronously demodulate 417 and output 419 the modulated-data signal asreceived data.

In use of the system as set forth in FIG. 3A, thetransmitter-generic-chip-code generator 101 generates thegeneric-chip-code signal, g₀(t). Message data are spread-spectrumprocessed by the EXCLUSIVE-OR device 103 with message-chip-code signal,g₁(t), from transmitter-message-chip-code generator 102. The combiner105 combines the generic-chip-code signal with thespread-spectrum-processed signal. The combined signal may be, forexample, a multilevel signal, which is generated by linearly adding thevoltage levels of the generic-chip-code signal and thespread-spectrum-processed signal, or by adding the voltage levels of thegeneric-chip-code signal with a plurality of spread-spectrum-processedsignals. The transmitter transmits on a carrier signal having a carrierfrequency, f_(o), the combined generic-chip-code signal and theplurality of spread-spectrum-processed signals. The CDMA signal istransmitted through the communications channel 110.

At the receiver, as shown in FIG. 3B, thegeneric-spread-spectrum-processing means, embodied as thereceiver-generic-chip-code generator 121, the generic mixer 123 and thegeneric-bandpass filter 125, cooperatively operate to recover thecarrier signal from the CDMA signal. Themessage-spread-spectrum-processing means, embodied as thereceiver-message-chip-code generator 122, the message mixer 124 and themessage-bandpass filter 126, cooperatively despread the CDMA signal asthe modulated-data signal. The receiver-message-chip-code generator 122preferably is synchronized to the replica of the generic-chip-codesignal from the receiver-generic-chip-code generator 121. A plurality ofreceiver-message-chip-code generators may be employed, synchronized tothe replica of the generic-chip-code signal. The synchronous means,embodied as the synchronous detector 127 synchronized to therecovered-carrier signal, demodulates the modulated-data signal asreceived data.

The received data are integrated and dumped by lowpass filter 128 andelectronic switch 130, under control of the bit synchronizer 129. Thebit synchronizer 129 preferably uses the replica of thegeneric-chip-code signal for synchronizing the integrate and dumpfunctions.

Spread Spectrum Geolocation

A spread spectrum code division multiple access (CDMA) communicationsand geolocation system and method for use over a communications channelis provided comprising at least one base station and a plurality ofremote units. The remote units may be mobile or in a fixed, stationarylocation. Message data are communicated between the base stations andthe remote units. Message data include, but are not limited to,digitized voice, computer data, facsimile data, video data, etc. Thebase station communicates base-message data to the plurality of remoteunits. A remote unit communicates remote-message data to the basestation. Base-message data are defined herein to be message dataoriginating from a base station, and remote-message data are definedherein to be message data originating from a remote unit. The followingdiscussion is of a preferred embodiment with the range between the basestation and remote unit being determined at the base station. The rolesof the base station and remote unit can be interchanged, as anequivalent to those skilled in the art, with the range being determinedat the remote unit.

In the exemplary arrangement shown in FIG. 5, a base station includesbase-spreading means, base-generic means, base-combiner means,base-transmitter means, and base antenna. The term “base” is used as aprefix to indicate an element is located at the base station, or that asignal originates from a base station.

The base-spreading means spread-spectrum processes the base-messagedata, d₁(t). The base-spreading means is embodied as abase-spread-spectrum modulator. The base-spread-spectrum modulator isshown as a message-chip-code generator 502 and an EXCLUSIVE-OR gate 503.The EXCLUSIVE-OR gate 503 is coupled to the message-chip-code generator502. The message-chip-code generator 502 uses a chip codeword forgenerating a chip-code sequence for spread-spectrum processingbase-message data, d₁(t). The chip-code sequence from message-chip-codegenerator 502 is spread-spectrum processed by modulo addition byEXCLUSIVE-OR gate 503. Many equivalent circuits can be used for thebase-spread-spectrum modulator, including but not limited to, productdevices for multiplying the chip-code sequence by the base-message data,matched filters and surface acoustic wave devices which have an impulseresponse matched to the chip-code sequence, as is well known to thoseskilled in the art.

The base-generic means generates a base-generic-chip-code signal. Theterm “generic” is used as a prefix to indicate that thegeneric-chip-code signal is an unmodulated, or low data rate,direct-sequence spread-spectrum signal, which can serve as a pilotchannel. The pilot channel allows a user to acquire timing, and providesa phase reference for coherent demodulation. The base-generic means isembodied as a base-generic-chip-code generator 501. Thebase-generic-chip-code generator 501 generates a base-generic-chip-codesignal, using a chip codeword commonly shared with all remote unitscommunicating with the base station. The message-chip-code generator 501is coupled to the base-generic-chip-code generator 502, for derivingcommon timing. Alternatively, a common clock can be used for providingthe timing signal to the message-chip-code generator 502 and thebase-generic-chip-code generator 501.

The base-combiner means combines the base-generic-chip-code signal withthe spread-spectrum-processed-base-message data, to generate a base-CDMAsignal. The base-combiner means is embodied as a base-combiner 505. Thebase combiner 505 is coupled to the base-generic-chip-code generator 501and the EXCLUSIVE-OR gate 503. The base combiner 505 linearly adds thebase-generic-chip-code signal with thespread-spectrum-processed-base-message data from EXCLUSIVE-OR gate 503.The resulting signal at the output of the base combiner 505 is a codedivision multiple access (CDMA) signal, denoted herein as the base-CDMAsignal. Selected variations of nonlinear combining also may be used, solong as the resulting base-CDMA signal can have its channels detected ata spread-spectrum receiver.

The base-transmitter means transmits the base-CDMA signal from the basestation to a remote unit. The base-transmitter means is embodied as asignal source 508 and product device 507. The product device 507 iscoupled between the base combiner 505 and the signal source 508. Thesignal source 508 generates a first carrier signal at a first carrierfrequency f₁. The base-CDMA signal, from the output of the base combiner505, is multiplied by the first carrier signal by product device 507.Other transmitting devices are well known in the art for putting adesired signal at a selected carrier frequency.

The base antenna 509 is coupled through an isolator 513 to thebase-transmitter means. The base antenna 509 radiates the base-CDMAsignal at the first carrier frequency.

As illustratively shown in FIG. 6, a remote unit includes a remoteantenna 511, remote-detection means, remote-spreading means,remote-combiner means, and remote-transmitter means. Each remote unitalso may include remote-generic means. The term “remote” is used as aprefix to indicate an element is located at a remote unit, or that asignal originates from the remote unit. The remote antenna 511 receivesthe base-CDMA signal radiated from the base station.

The remote-detection means is coupled to the remote antenna 511. Theremote-detection means detects the base-generic-chip-code signalembedded in the base-CDMA signal. Using thedetected-base-generic-chip-code signal, the remote-detection meansrecovers the base-message data communicated from the base station. Aremote unit can retransmit the detected-base-generic-chip-code signal,or optionally, can have remote-generic means generate a differentremote-generic-chip-code signal.

In FIG. 6, the remote-detection means is embodied as a product device536, bandpass filter 537, acquisition and tracking circuit 538,generic-chip-code generator 539, message-chip-code generator 541,product device 542, bandpass filter 543, data detector 544, lowpassfilter 545, and bit synchronizer 540. As is well known in the art, otherdevices and circuits can be used for the same function, including butnot limited to, matched filters, surface acoustic wave devices, etc.This circuit acquires and tracks the base-generic-chip-code signalembedded in the base-CDMA signal. The base-CDMA signal is received atremote antenna 511, and passes through isolator 534 and power splitter535. The base-generic-chip-code signal is detected using product device536, bandpass filter 537, acquisition and tracking circuit 538 andgeneric-chip-code generator 539. The function of this circuit is asdescribed in the previous section. The detected-base-generic-chip-codesignal is used to recover the base-message data embedded in thebase-CDMA signal, using message-chip-code generator 541, product device542, bandpass filter 543, data detector 544, lowpass filter 545, andsynchronizer 540. The data detector 544 may operate coherently ornoncoherently. The detected base-message data is outputted as detecteddata, d_(R1)(t). If the base-generic-chip-code signal is to be combinedas part of the remote-CDMA signal, then generic-chip-code generator 546is not required, since the base-generic-chip-code signal is available atthe output of the generic-chip-code generator 539. If aremote-generic-chip-code signal, which is different from thebase-generic-chip-code signal, is to be used, then the generic-chip-codegenerator 546 can be used for generating the remote-generic-chip-codesignal. In the latter case, the remote-generic-chip-code signal isclocked or synchronized with the detected base-generic-chip-code signal.For purposes of discussion, the remote-generic-chip-code signal isconsidered to be sent from the remote unit to the base station, with theunderstanding that the remote-generic-chip-code signal can be identicalto, or one and the same as, the detected base-generic-chip-code signal.

The remote-spreading means spread-spectrum processes remote-messagedata. The remote-spreading means is embodied as a remote-spread-spectrummodulator. The remote-spread-spectrum modulator is shown as amessage-chip-code generator 548 and an EXCLUSIVE-OR gate 547. TheEXCLUSIVE-OR gate 547 is coupled to the message-chip-code generator 548.The message-chip-code generator 548 uses a chip codeword for generatinga chip-code sequence for spread-spectrum processing remote-message data,d₂(t). The chip-code sequence from message-chip-code generator 548 isspread-spectrum processed by modulo addition by EXCLUSIVE-OR gate 547.Many equivalent circuits can be used for the remote-spreading means,including but not limited to, product devices for multiplying thechip-code sequence by the base-message data, matched filters and surfaceacoustic wave devices, as is well known to those skilled in the art.

The remote-generic-chip-code signal and thespread-spectrum-processed-remote-message data are combined by theremote-combiner means, as a remote-CDMA signal. The remote-combinermeans is embodied as a remote-combiner 549. The remote combiner 549 iscoupled to the EXCLUSIVE-OR gate 547, and the remote-generic-chip-codegenerator 546, or alternatively to the generic-chip-code generator 539.The remote combiner 549 linearly adds the remote-generic-chip-codesignal with the spread-spectrum-processed-remote-message data fromEXCLUSIVE-OR gate 547. The resulting signal at the output of the remotecombiner 549 is a code division multiple access (CDMA) signal, denotedherein as the remote-CDMA signal. Selected variations of nonlinearcombining also may be used, so long as the resulting remote-CDMA signalcan have its channels detected at a spread-spectrum receiver.

The remote unit also includes the remote-transmitter means fortransmitting the remote-CDMA signal from the remote unit to the basestation. The remote-transmitter means is embodied as a signal source 551and product device 550. The product device 550 is coupled between theremote combiner 549 and the signal source 551. The signal source 551generates a carrier signal at a second carrier frequency f₂. Theremote-CDMA signal, from the output of the remote combiner 549, ismultiplied by the second carrier signal by product device 550. Othertransmitting devices are well known in the art for putting a desiredsignal at a selected carrier frequency. The second carrier frequency maybe the same as, or different from, the first carrier frequency.

The remote antenna 511 is coupled through an isolator 534 to theremote-transmitter means. The remote antenna 511 radiates theremote-CDMA signal at the second carrier frequency.

Each of the base stations further includes base-detection means andrange means. The base-detection means is coupled to the base antenna 509through isolator 513 and power splitter 515. The base detection meansdetects the remote-generic-chip-code signal embedded in the remote-CDMAsignal. The base-detection means, as illustrated in FIG. 5, may beembodied as a base detector which may includes a product device 523,bandpass filter 525, acquisition and tracking circuit 531,generic-chip-code generator 521, message-chip-code generator 522,product device 524, bandpass filter 526, data detector 527, lowpassfilter 528, and bit synchronizer 529. As is well known in the art, thebase detection means may be embodied with other devices and circuitswhich perform the same function, including but not limited to, matchedfilters, surface acoustic wave devices, etc. This circuit acquires andtracks the remote-generic-chip-code signal embedded in the remote-CDMAsignal. The remote-CDMA signal is received at base antenna 509, andpasses through isolator 513 and power splitter 515. Theremote-generic-chip-code signal is detected using product device 523,bandpass filter 525, acquisition and tracking circuit 531 andgeneric-chip-code generator 521. The function of this circuit is aspreviously described. The detected-remote-generic-chip-code signal isused to recover the remote-message data embedded in the remote-CDMAsignal, using message-chip-code generator 522, product device 524,bandpass filter 526, data detector 527, lowpass filter 528, bit andsynchronizer 529. The data detector 527 may operate coherently ornoncoherently. The detected remote-message data is outputted as detecteddata, d_(R2)(t). Thus, the base detector recovers, using thedetected-remote-generic-chip-code signal, the remote message datacommunicated from the remote unit.

Using the detected-remote-generic-chip-code signal and thebase-generic-chip-code signal, the range means determines a range delaybetween the remote unit and the base station. The range means isembodied as a range delay device 530, which can compare the timingbetween the base-generic-chip-code signal from the generic-chip-codegenerator 501, with the detected remote-generic-chip-code signal fromthe generic-chip-code generator 521.

The present invention may include further the steps of spread-spectrumprocessing the base-message data; generating a base-generic-chip-codesignal; combining the base-generic-chip-code signal with thespread-spectrum-processed-base-message data, thereby generating abase-CDMA signal; transmitting the base-CDMA signal from the basestation to the remote unit; detecting the base-generic-chip-code signalembedded in the base-CDMA signal; recovering, using thedetected-base-generic-chip-code signal, the base-message data;spread-spectrum processing remote-message data; generating, using thedetected-generic-chip-code signal and thespread-spectrum-processed-remote data, a remote-CDMA signal;transmitting the remote-CDMA signal from the remote unit to the basestation; detecting the remote-generic-chip-code signal embedded in theremote-CDMA signal; recovering, using thedetected-remote-generic-chip-code signal, the remote-message data; anddetermining, using the detected-remote-generic-chip-code signal and thebase-generic-chip-code signal, a range delay between the remote unit andthe base station.

In use, the base station spread-spectrum processes the base-message datawith a message-chip-code signal, and combines thespread-spectrum-processed-base-message data with abase-generic-chip-code signal. The combined signal is a base-CDMA signalwhich is transmitted over a communications channel to at least oneremote unit.

The remote unit receives the base-CDMA signal, detects thebase-generic-chip-code signal embedded in the base-CDMA signal, and usesthe detected-base-generic-chip-code signal to recover the base-messagedata embedded in the base-CDMA signal.

The detected base-generic-chip-code signal is relayed as aremote-generic-chip-code signal, or is used to set the timing for adifferent remote-generic-chip-code signal, which is sent from the remoteunit to the base station. The remote unit spread-spectrum processes theremote-message data with a remote-chip-code signal, and combines thespread-spectrum-processed-remote-message data with theremote-generic-chip-code signal as a remote-CDMA signal. The remote-CDMAis sent over the communications channel to the base station.

At the base station, the remote-generic-chip-code signal is detectedfrom the remote-CDMA signal, and the detected remote-generic-chip-codesignal is used to detect the remote-message data embedded in theremote-CDMA signal. Additionally, the detected remote-generic-chip-codesignal is compared with the base-generic-chip-code signal in arange-delay circuit, to determine the range of the remote unit from thebase station. Effectively, the range between the remote unit and thebase station is a function of the timing between sending a sequence ofthe chip codeword which generated the base-generic-chip-code signal, andreceiving the sequence generated by the chip codeword which generatedthe remote-generic-chip-code signal.

The concept of using a radio frequency (RF) signal to determine range iswell known in the art. The RF signal is subject to a fixed rate ofpropagation, 3×10⁸ meters/sec. The RF signal leaves a transmitter sometime before it reaches a receiver. A particular sequence of thebase-generic-chip-code signal and remote-generic-chip-code signal areused as a mark in time. The difference in time of the sequence of thebase-generic-chip-code signal as seen at the receiver of the remoteunit, from that present at the transmitter of the base station, isrelated directly to distance between the base station and remote unit.Similarly, the difference in time of the sequence of theremote-generic-chip-code signal as seen at the receiver of the basestation form that present at the transmitter of the remote unit, isrelated directly to distance between the remote unit and base station.

The use of the base-generic-chip-code signal and theremote-generic-chip-code signal is a common type of echo rangemeasurement method that is used in radar systems. Many radar systemssimply employ a pulse of RF energy and then wait for a return of aportion of the energy due to the pulse being reflected from objects. Theradar marks time from the instant of pulse transmission until itsreturn. The time required for the pulse to return is a function of thetwo-way range to the object. The range is easily determined from thesignal propagation speed.

The spread-spectrum signals of the present invention are subject to thesame distance/time relationship. The spread-spectrum signal of thepresent invention has an advantage in that its phase is easilyresolvable. The basic resolution of a sequence of a base-chip-codesignal or a remote-chip-code signal is one code chip. Thus, the higherthe chip rate, the better the measurement capability. Thus, at a chiprate of 10 Mchips/sec, a basic range resolution is 10⁻⁷ seconds, or 30meters. Additional delays may be encountered in the circuitry of theremote unit. These delays can be compensated at the base station, whendetermining the distance between the base station and the remote unit.

Orthogonal Code Synchronization

The present invention may also be embodied as a system and method usingorthogonal codes and knowledge of the distance to the mobile terminal toadjust and align the phase of the information channel to achieveorthogonality at the base station antenna.

The system for using orthogonal codes and knowledge of the distance tothe mobile terminal to achieve orthogonality at the base station antennacomprises a plurality of mobile terminals and a base station. Each ofthe plurality of mobile terminals includesremote-spread-spectrum-processing means, remote-pilot means, combiningmeans, remote-transmitting means, and code phase adjustment means.

The remote-spread-spectrum-processing means and the remote-pilot meansare coupled to the combining means. The remote-transmitting means iscoupled to the combining means.

The base station includes receiving means, first-base-pilot means,second-base-pilot means, first delay means, second delay means,correlator means, tracking means, range delay means, andbase-transmitting means.

The remote-spread-spectrum-processing means processes remote-messagedata using a pseudo-noise code. The remote-pilot means generates aremote-pilot signal. The combining means combines the remote-pilotsignal with the spread-spectrum-processed-remote-message data togenerate a remote composite signal. The remote composite signal has theremote-pilot signal and at least one remote-user-information channel.The remote-transmitting means transmits the remote composite signal tothe base station on a reverse channel of the duplex radio channel.

At the base station, the receiving means receives the remote compositesignal. The first-base-pilot means generates a base pilot signal. Thesecond-base-pilot means generates a base-pilot-reference-signal. Thefirst delay means generates an on-time, an early, and a late version ofthe base-pilot-reference signal. The second delay means generates aninformation reference signal. The information reference signal issynchronized with the on-time version of the base-pilot-referencesignal. Correlator means multiplies the remote composite signal with theon-time, the early, and the late versions of the base-pilot-referencesignal to correlate out an on-time, an early, and a late version of theremote-pilot signal, respectively. Correlators means also multiplies theremote composite signal with the information reference signal tocorrelate out the remote-user-information channel.

Tracking means tracks the phase of the remote-pilot signal and, inresponse to a peak in the remote-pilot signal, outputs an acquisitionsignal. The acquisition signal signifies synchronization of theremote-pilot signal and the base-pilot-reference-signal.

In response to the acquisition signal, the range delay means calculatesa phase difference between the base pilot signal and thebase-pilot-reference-signal to determine the range between the mobilestation and the base station. The base-transmitting means transmits therange from the base station to the mobile terminal over a forwardchannel of the duplex radio channel.

In response to the range information received from the base station, thecode phase adjustment means at the mobile terminal adjusts the phase ofthe pseudo-noise code to determine the arrival time of thespread-spectrum-processed-remote-message data at the base station.

When adjusting the phase of the pseudo-noise code, theremote-spread-spectrum-processing means may adjust the pseudo-noise codein increments of a code chip. A base station processor compares signalstrength levels of the spread-spectrum-processed-remote-message data asthe mobile terminal adjusts the pseudo-noise code. In response to a codechip increment that maximizes performance, the base station calibrates arelationship between the remote-pilot signal and thespread-spectrum-processed-remote-message data with the code chipincrement.

The spread spectrum CDMA cellular radio communications system mayfurther comprise base-spreading means and base-combining means. Thebase-spreading means spread-spectrum processes base-message data. Thebase-spreading means may include means for processing base-message datafor a particular mobile terminal with a selected chip code. Thebase-combining means combines the spread-spectrum-processed-base-messagedata and the base pilot signal as a composite base signal. The compositebase signal includes a common-shared-spread-spectrum-pilot signal and atleast one specific spread spectrum user information channel for eachmobile terminal. The spreading code of each of thecommon-shared-spread-spectrum-pilot signal and the specific spreadspectrum user information channel may contain an orthogonal codeelement. The remote-pilot signal may be slaved to thecommon-shared-spread-spectrum-pilot signal as a reference for phase andtiming of the remote-pilot signal.

The remote-pilot signal and a spreading code of theremote-user-information channel of each mobile terminal may contain anorthogonal code element. Further, the remote-user-information channelmay be synchronized to the remote-pilot signal.

The system may further comprise base station delay lock loop means forgenerating an error signal and for tracking the remote-pilot signal. Themobile terminal, responsive to receiving the error signal from the basestation, adjusts an orthogonal pseudo-noise code phase to compensate forchanges in range as the mobile terminal moves within the cell.

More specifically, the mobile terminal of the present inventioncomprises a remote user data source, a first orthogonal code setgenerator, a first noise-like code generator, a remote pilot datasource, a signal combiner, a first modulo-two adder, a second modulo-twoadder, a third modulo-two adder, a fourth modulo-two adder, a modulator,antenna means, a code phase adjuster, and a processor.

The first modulo-two adder is coupled to the remote user data source andto the first orthogonal code set generator. The second modulo-two adderis coupled to an output of the first modulo-two adder and to the firstnoise-like code generator. The third modulo-two adder is coupled to thefirst orthogonal code set generator and to the remote pilot data source.The fourth modulo-two adder is coupled to an output of the thirdmodulo-two adder and to the first noise-like code generator. The signalcombiner is coupled to the fourth modulo-two adder and to the secondmodulo-two adder. The modulator is coupled to the signal combiner. Thecode phase adjuster is coupled to the first orthogonal code setgenerator and to the first noise-like code generator. The processor iscoupled to the code phase adjuster.

The remote user data source generates a user data signal. The firstorthogonal code set generator generates a first orthogonal code and afirst remote pilot code. The first modulo-two adder spread spectrumprocesses the user data signal with the first orthogonal code togenerate a spread signal. The first noise-like code generator generatesa first pseudo-noise code. The second modulo-two adder processes thespread signal with the first pseudo-noise code to generate a spreadspectrum user data signal.

The remote pilot data source generates a pilot data signal. The pilotdata signal may be comprised of all ones. Alternatively the remote pilotdata source may generate a pilot data signal that is comprised of allzeros.

The third modulo-two adder spread spectrum processes the pilot datasignal with the first remote pilot code to generate a spread pilotsignal. The fourth modulo-two adder processes the spread pilot signalwith the first pseudo-noise code to generate a remote spread spectrumpilot data signal.

The signal combiner combines the remote spread spectrum pilot datasignal and the spread spectrum user data signal as a combined spreadspectrum data signal. The modulator modulates the combined spreadspectrum data signal onto a carrier as a combined spread spectrummodulated data signal. Antenna means transmit the combined spreadspectrum modulated data signal on a reverse channel of a duplex radiochannel. Antenna means also receives a composite spread-spectrummodulated carrier signal transmitted from the base station on a forwardchannel of the duplex radio channel. The composite spread-spectrummodulated carrier signal on the forward channel has acommon-shared-spread-spectrum-pilot signal and a specific spreadspectrum user information channel for the mobile terminal.

The code phase adjuster adjusts, responsive to an input from theprocessor and to the common-shared-spread-spectrum-pilot signal, a phaseof the first orthogonal code to adjust the time of arrival of the spreadspectrum user data signal at the base station. This phase adjustmentadjusts the time of arrival to be orthogonal with other arriving spreadspecturm user data signals. The phase of the first remote pilot code isslaved to the common-shared-spread-spectrum-pilot signal to enable thebase station to determine the range between the base station and themobile terminal using round trip delay. The processor generates theprocessor input and stores the range between the base station and themobile terminal.

The code phase adjuster may further adjust the phase of the firstorthogonal code to have a same phase as the first pseudo-noise code. Thelength of the first pseudo-noise code is an integer multiple of a lengthof the first orthogonal code. The code phase adjuster may further shift,responsive to acquisition, a phase of the first remote pilot code to besynchronous with the spread spectrum user data signal.

The mobile terminal of the present invention may further comprise apower splitter, a second orthogonal code set generator, a secondnoise-like code generator, a mode control and acquisition device, aclock pulse generator, a fifth modulo-two adder, a sixth modulo-twoadder, a first delay device, a second delay device, a firstmultiplier/correlator, a second multiplier/correlator, a thirdmultiplier/correlator, a fourth multiplier/correlator, a delay lockloop, and a phase lock loop oscillator.

The mode control and acquisition device is coupled between the secondorthogonal code set generator and the second noise-like code generator.The clock pulse generator is coupled to the mode control and acquisitiondevice and to the first orthogonal code set generator and to the firstnoise-like code generator. The fifth modulo-two adder is coupled to thesecond orthogonal code set generator and to the second noise-like codegenerator. The sixth modulo-two adder is coupled to the secondorthogonal code set generator and to the second noise-like codegenerator. The first delay device is coupled to the fifth modulo-twoadder. The first, the second, and the third multiplier/correlators areeach coupled to the power splitter and to the first delay device. Thesecond delay device is coupled to the sixth modulo-two adder and to theprocessor. The fourth multiplier/correlator is coupled to the seconddelay device and to the power splitter. The delay lock loop is coupledto the second and third multiplier/correlators. The phase lock looposcillator is coupled to the first multiplier/correlator.

The power splitter separates the composite spread-spectrum modulatedcarrier signal into a pilot channel and a data channel. The secondorthogonal code set generator generates, responsive to a command fromthe base station, a plurality of locally generated pilot codes, any oneof which, or any plurality of which, may be generated and/or used at anygiven time.

The second noise-like code generator generates a second pseudo-noisecode. The mode control and acquisition device receives timinginformation from the base station and generates a clock for the secondorthogonal code set generator and for the second noise-like codegenerator. The mode control and acquisition device also generates asynchronization signal. The clock pulse generator provides a synchronousclock signal. The clock pulse generator may have its own oscillator ormay lock onto the clock from the mode control and acquisition device.

The fifth modulo-two adder combines a first locally generated pilot codeand the second pseudo-noise code to form a first localspread-spectrum-pilot-reference signal. The sixth modulo-two addercombines a specified orthogonal code and the second pseudo-noise code toform a first local spread spectrum information reference signal. Thefirst delay device delays, responsive to the processor, the first localspread-spectrum-pilot-reference signal to generate an on-time, an early,and a late version of the first local spread-spectrum-pilot-referencesignal.

The first, the second, and the third multiplier/correlators multiply thecomposite-spread-spectrum-modulated-carrier signal with the on-time, theearly, and the late versions of the first localspread-spectrum-pilot-reference signal to correlate out an on-time, anearly, an a late version of the common-shared-spread-spectrum-pilotsignal, respectively. The second delay device provides an informationreference signal synchronized with the on-time version of the firstlocal spread-spectrum-pilot-reference signal. The fourthmultiplier/correlator multiplies the composite spread-spectrum modulatedcarrier signal with the first local spread spectrum informationreference signal to correlate out the specific spread spectrum userinformation channel. The delay lock loop tracks the phase of theincoming common-shared-spread-spectrum-pilot signal and outputs,responsive to a correlation peak, a clock signal and an acquisitionsignal to the mode control and acquisition device. The phase lock looposcillator centers on the correlation peak and provides a coherentcarrier reference to a local data detector and to the delay lock loop.

Using the system just described, the base station can determine therange between the base station and the mobile terminal by measuring thecode phase difference between the common-shared-spread-spectrum-pilotsignal and the first remote pilot code. Thecommon-shared-spread-spectrum-pilot signal may contain an orthogonalelement. The first remote pilot code may also contain an orthogonal codeelement.

In the spread spectrum CDMA digital cellular radio system of the presentinvention, the system also includes a base station. The base stationcomprises a base user data source, a first orthogonal code setgenerator, a first noise-like code generator, a first modulo-two adder,a second modulo-two adder, a system data source, system data spreadingmeans, a base pilot data source, pilot data spreading means, a signalcombiner, a modulator, antenna means, pilot-reference-signal generatingmeans, a clock pulse generator, range delay means, and a processor.

The first modulo-two adder is coupled to the base user data source andto the first orthogonal code set generator. The second modulo-two adderis coupled to an output of the first modulo-two adder and to the firstnoise-like code generator. The signal combiner is coupled to the pilotdata signal spreading means, the system data spreading means, and thesecond modulo-two adder. The modulator is coupled to the signalcombiner. The clock pulse generator is coupled to the first orthogonalcode set generator and to the first noise-like code generator. Rangedelay means is coupled to an output of the pilot data spreading meansand to an output of the pilot-reference-signal generating means.

The base user data source generates a base user data signal. The firstorthogonal code set generator generates a first orthogonal code and afirst base pilot code. The first modulo-two adder spread spectrumprocesses the base user data signal with the first orthogonal code togenerate a spread signal. The first noise-like code generator generatesa first pseudo-noise code. The second modulo-two adder processes thespread signal with the first pseudo-noise code to generate a spreadspectrum user data signal. The system data source generates system datato be transmitted to the plurality of mobile terminals. The system dataspreading means spread spectrum processes the system data. The basepilot data source generates a base pilot data signal. The pilot datasignal spreading means spread spectrum processes the base pilot datasignal with the first base pilot code as acommon-shared-spread-spectrum-pilot signal.

The signal combiner combines the common-shared-spread-spectrum-pilotsignal, the spread spectrum system data, and the spread spectrum userdata signal as a combined spread spectrum data signal. The modulatormodulates the combined spread spectrum data signal onto a carrier as acombined spread spectrum modulated data signal. Antenna means transmitsthe combined spread spectrum modulated data signal. Antenna means alsoreceives a plurality of composite spread-spectrum modulated carriersignals transmitted from a plurality of mobile terminals, respectively.Each composite spread-spectrum modulated carrier signal has areceived-remote-spread-spectrum-pilot signal and an information channelfor each mobile terminal.

The pilot-reference-signal generating means generates a pilot-referencesignal. The clock pulse generator maintains system-wide time. Rangedelay means calculates a phase difference between the pilot-referencesignal and the common shared spread-spectrum pilot signal as a firstvalue. The processor stores the first value and provides, using thefirst value, a processor output representing round trip delay to themobile terminal.

The pilot data signal spreading means may comprise a third modulo-twoadder and a fourth modulo-two adder. The third modulo-two adder iscoupled to the first orthogonal code set generator and to the base pilotdata source. The fourth modulo-two adder is coupled to an output of thethird modulo-two adder and to the first noise-like code generator.

The third modulo-two adder spread spectrum processes the pilot datasignal with the first base pilot code to generate a spread pilot signal.The fourth modulo-two adder processes the spread pilot signal with thefirst pseudo-noise code to generate acommon-shared-spread-spectrum-pilot signal.

The system data spreading means may comprise a fifth modulo-two adderand a sixth modulo-two adder. The fifth modulo-two adder is coupled tothe first orthogonal code set generator and to the system data source.The sixth modulo-two adder is coupled to an output of the fifthmodulo-two adder and to the first noise-like code generator.

The first orthogonal code set generator generates a second orthogonalcode. The fifth modulo-two adder spread spectrum processes the systemdata with the second orthogonal code to generate a spread-spectrum-datasignal. The sixth modulo-two adder processes the spread-spectrum-datasignal with the first pseudo-noise code to generate aspread-spectrum-system-data signal.

The base station may further comprise a power splitter, a secondorthogonal code set generator, a second noise-like code generator, amode control and acquisition device, a seventh modulo-two adder, aneighth modulo-two adder, a first delay device, a second delay device, afirst multiplier/correlator, a second multiplier/correlator, a thirdmultiplier/correlator, a fourth multiplier/correlator, a delay lock loopand a phase lock loop oscillator.

The mode control and acquisition device is coupled between the secondorthogonal code set generator and the second noise-like code generator.The seventh modulo-two adder is coupled to the second orthogonal codeset generator and to the second noise-like code generator. The eighthmodulo-two adder is coupled to the second orthogonal code set generatorand to the second noise-like code generator. The first delay device iscoupled to the seventh modulo-two adder. The second delay device iscoupled to the eighth modulo-two adder and to the processor. The first,the second, and third multiplier/correlators are each coupled to thepower splitter and to the first delay device. The fourthmultiplier/correlator is coupled to the second delay device and to thepower splitter. The delay lock loop is coupled to the second and thirdmultiplier/correlators. The phase lock loop oscillator is coupled to thefirst multiplier/correlator.

The power splitter separates thecomposite-spread-spectrum-modulated-carrier signal into a pilot channeland a data channel. The second orthogonal code set generator generates athird orthogonal code. The second noise-like code generator generates asecond pseudo-noise code. The mode control and acquisition deviceprovides clock and control signals.

The seventh modulo-two adder combines an assigned pilot orthogonal codeand the second pseudo-noise code to form the firstspread-spectrum-pilot-reference signal. The eighth modulo-two addercombines an assigned data orthogonal code and the second pseudo-noisecode to form the first spread spectrum data reference signal.

The first delay device delays, responsive to the processor, the firstspread-spectrum-pilot-reference signal to generate an on-time, an early,and a late version of the first spread-spectrum-pilot-reference signal.The first, the second, and the third multiplier/correlators multiply thecomposite-spread-spectrum-modulated-carrier signal with the on-time, theearly, and the late versions of the firstspread-spectrum-pilot-reference signal to correlate out an on-time, anearly, and a late version of the received-remote-spread-spectrum-pilotsignal, respectively.

The second delay device provides an information reference signalsynchronized with the on-time version of the firstspread-spectrum-pilot-reference signal. The fourth multiplier/correlatormultiplies the composite spread-spectrum modulated carrier signal withthe information reference signal to correlate out the informationchannel.

The delay lock loop tracks the phase of the received remotespread-spectrum pilot signal. In response to a correlation peak, thedelay lock loop outputs a clock signal and an acquisition signal to themode control and acquisition device. The phase lock loop oscillatorprovides a coherent carrier reference to a local data detector and tothe delay lock loop.

Using the system just described, the base station can determine therange to each mobile terminal by measuring the code phase differencebetween the common-shared-spread-spectrum-pilot signal and thereceived-remote-spread-spectrum-pilot signal.

The mobile terminal may adjust, responsive to the round trip delay, acode phase of the information channel of eachcomposite-spread-spectrum-modulated-carrier signal to coincide with aspecific time mark as the composite-spread-spectrum-modulated-carriersignal arrives at the base station. The base station may set thespecific time mark at an absolute time value to satisfy cellorthogonality criteria.

As illustratively shown in FIG. 7 a and FIG. 7 b, a mobile terminal ofthis invention may include a remote antenna 727, a remote terminal datasource 700, a remote pilot data source 701, remote orthogonal code setgenerators 702, 740, noise-like code generators 703, 741, six modulo-twoadders 710-715, a combiner 716, a radio frequency modulator/translator720, a clock pulse generator 730, a processor 732, a code phase adjuster731, a mode control and acquisition device 733, four bandpass filters754, 755, 756, 757, a bit synchronizer 759, a coherent detector 758, anintegrate and dump circuit 760, a delay lock loop 751, two delayelements 752, 753, four multiplier correlators 725, 726, 728, 729, aphase lock loop oscillator 750, a power splitter 722, a diplexer 721,and a carrier generator 719. FIG. 7 also shows processor input/output771, user data input 770, and radio frequency input/output 773.

The remote terminal data source 700 of FIG. 7 is the informationpresented to the mobile terminal by the remote user. This informationcan be voice, data, fax or any other form of information that the userdesires to send over his mobile terminal to another user, machine orsystem. The processor 732 also generates messages for use by the radiosystem or other distant user, and provides these messages to the remoteuser data source through the user data input 770 where the messages aremultiplexed with the user data. The remote user data source presents themultiplexed user data signal to the modulo-two-adder 710 where anassigned orthogonal code, operating at a much higher bit rate than theuser data, is superimposed on the user data signal. The orthogonal codespreads the user data signal so that several similar signals may occupythe same spectrum and be recovered at the base station. The spreadsignal is superimposed by an additional PN code by the modulo-two-adder711 to make the resulting spread spectrum signal more like random noise.The PN code is generated by the noise-like code generator 703. Thespread spectrum user data signal is combined with the spread spectrumpilot data signal in the combiner 716. The combined spread spectrum datasignal is modulated on the carrier frequency, w_(c), by the radiofrequency modulator/translator 720. The spread spectrum modulated datasignal is routed to the remote antenna 727 through the diplexer 721,which allows the remote antenna 727 to be used for both transmitting andreceiving. The remote antenna 727 transmits thecomposite-spread-spectrum-modulated-carrier signal over the air to thebase station antenna means where it is received. Many otherimplementations are possible and would be obvious to one skilled in theart. For instance, the orthogonal code and the noise-like code could becombined before they are added to the data. The modulation could be doneat baseband by using quadrature carrier components and combining thecomponents at radio frequencies. Different PN codes could be used on thedifferent quadrature components to add to the randomness of thecomposite signal. These are well known techniques to those schooled inthe technology.

The orthogonal code set generator 702 can generate any code that belongsto the predetermined set of codes and is directed to generate a specificcode by the processor 732. The processor 732 in turn receives itsdirection through input/output 771 from the base station control meansover the control channel. The orthogonal code set generator 702 sets upand generates the assigned code, as described above, and said orthogonalcode is used to spread the user data signal in modulo-two-adder 710. Theorthogonal code set generator 702 also generates a second assignedorthogonal code that is used to spread the pilot data signal inmodulo-two-adder 712. The phases of these codes are adjustedindependently, but the clock rate is the same for both codes. Afteracquisition, for all modes of operation, the clock pulse generator 730is slaved to the incoming, or base station, timing and clock receivedfrom the mode control and acquisition device 733. During acquisitionmode, the clock pulse generator 730 uses an internal oscillator thatoperates at approximately the expected rate that will be received fromthe base station. This internal oscillator can be set to be slightlyhigher or lower in clock rate to allow the scanning of the incomingcomposite spread signal. Upon acquisition, the mode control andacquisition device 733 provides a clock synchronization signal to theclock pulse generator 730.

The phase of the orthogonal code can be adjusted to the same phase asthe incoming pilot code from the base station. This makes thetransmitted user pilot signal look like a reflection from the mobileterminal and the base station can measure the round trip delay to eachspecific mobile terminal. This round trip delay, measured in code chips,is sent to the mobile terminal and stored in the processor 732. One-halfof the round trip delay is the distance between the mobile terminal andthe base station measured in code chips. The accuracy of the distancecan be improved by using increments of one-eighth or one tenth of thechip times and determining the peak output power from the correlator atthe base station and then sending the delay time to the mobile terminalwith a fraction of a chip accuracy.

The mobile terminal has the ability to adjust the phase of theorthogonal code in fractions of a chip, for instance one-eighth,one-tenth or one-sixteenth, as directed by the code phase adjuster 731which, with the assistance of the processor 732, determines the phase ofthe received pilot signal and translates that into the proper initialstates for the remote pilot spreading code.

To cause the transmitted, reverse link, spread spectrum user data signalto be orthogonal with the other transmitted spread spectrum user datasignals, as the signals arrive at the base station, the phase of thecode transmitted by each user must be adjusted to compensate for thedifferent path lengths, or distances, to each of the individual users.Each of the mobile terminals has stored in its memory the distance tothe base station. With this information the processor 732 determines thephase adjustment required to have the spread spectrum user data signalarrive at the base station at the specified time. The code phaseadjuster 731 then provides the initial code settings for the orthogonalcode set generator 702 and starts the generator at the proper time. Thebase station user data channel calibration detector detects the errorvoltage to maximize the correlation output power, in fractions of achip, and sends a correction signal to the mobile terminal to provideincremental adjustments to the user data orthogonal code phase to finetune the relative position of the transmitted signals. These incrementaladjustments, with the pilot tracking error signal, compensate for normalmovement of the mobile terminal and track the mobile terminal as itmoves in the region.

Very rapid changes in code phases will require the reacquisition of thedata signal by repeating the range measurement technique, using thepilots as described above. The noise-like code generator 703 is phaseadjusted by the code phase adjuster 731 to have the same phase as theorthogonal code set generator 702. Since the noise-like PN code is muchlonger than the orthogonal code, the orthogonal code and the noise-likePN code are adjusted to appear to start at the same time and theorthogonal code will repeat many times during one cycle of thenoise-like PN code and they will end at the same time. Therefore, theyboth start at the beginning of an epoch, where the epoch is the lengthof the noise-like PN code. The length of the orthogonal code is an eveninteger of the longer noise-like PN code. The same noise-like PN code isused for all users and becomes a digital carrier for all the user datasignals. When the noise-like PN code is synchronously detected, it hasno impact on the discrimination between the different orthogonal codes.

The process described above results in the transmitted user data signaland the transmitted pilot having different absolute phases with respectto the system time reference. Therefore, the pilot spread spectrumsignals cannot be orthogonal to the user data signals. This means,assuming every user terminal also has a pilot signal, if half of thesignals appear as random noise and the other half do not contributeinterference, the interference has been reduced by 3 db. The pilot datafrom the remote pilot data source 701 can be all zeros, all ones oractually have a low data rate information signal input on the pilotchannel. Assuming an “all ones” input for the remote pilot data source701, the pilot channel only transmits the addition of the orthogonalcode selected for the pilot and the noise-like PN code.

As stated previously, the phase and timing of the remote pilot areslaved to the incoming pilot from the base station. The pilot is slavedto appear to have no delay as it passes through the mobile terminal.This is a key feature of this invention and allows the base station toaccurately measure the round trip delay. The base station provides thisround trip delay information to the mobile terminal which uses it duringacquisition to adjust the phase of the transmitted user data signal sothat the base station can quickly acquire the user data signal in theorthogonal mode of operation. Since the mobile terminal uses the samecarrier for both the pilot and user data signals, the pilot carrierphase is used to coherently detect the user data. As stated above, afteracquisition the distance information from the pilots is not necessaryduring the normal transfer of data mode. Therefore, the mobile terminalincludes a mode, used after acquisition has occurred, where the pilotcode phase is shifted to have the same phase as the user data channel.In this mode the pilots are also orthogonal if the assigned pilot codesare members of the orthogonal code set. This feature of the presentinvention nearly doubles the system capacity again. This also means thepilot can be transmitted at relatively high power levels since the pilotdoes not contribute interference to the other signals. It does mean,though, that the number of users has been reduced if the limitation oncapacity is caused by a limited number of orthogonal codes and notprocessing gain. Since this feature is controlled from the base station,the base station can make the assessment as to which mode will give thebest performance with the largest capacity and act accordingly.

The pilot data is modulo-two added to the code assigned to the pilot inthe adder 712, which results in a spread spectrum reverse link pilotsignal. This signal also has a noise-like PN signal added to it in theadder 713, where the pilot signal is made to appear more like a randomnoise spread spectrum signal. This noise-like spread spectrum pilotsignal is combined with the spread spectrum user data signal in thecombiner 716 to form the composite spread spectrum signal that is thenmodulated onto the carrier in the modulator/translator 720. Thismodulated composite spread spectrum signal passes through the diplexer721 and on to the antenna 727.

The antenna 727 also receives the composite spread spectrum signaltransmitted from the base station. This signal is passed through thediplexer 721 where it is isolated from the transmitted signal, and isdivided in the power splitter 722 into a pilot channel and a datachannel. The pilot channel may use three different correlators to trackthe carrier and spreading code; these three correlators are composed ofmultiplier/correlators 726, 728, 729 plus integrator/bandpass filters754, 756, 757. The delay lock loop 751 tracks the phase of the incomingcode and keeps the local pilot code, generated by modulo-two adding thelocally generated orthogonal and noise-like codes, in synchronizationwith the composite spread spectrum signal transmitted by the basestation. The local pilot code is multiplied with the incoming compositespread signal in the multiplier/correlators 726, 728, 729. The delayelement 752 delays the reference pilot inputs to themultiplier/correlators 726, 728, 729 in such a way as to yield anon-time, an early, and a late version of the reference pilot,respectively. The early and late signals, multiplied bymultiplier/correlators 728, 729, respectively, are used by the delaylock loop 751 to track the incoming signal. When the codes are phasealigned with the inputs to the three multiplier/correlators 726, 728,729 coming from the power splitter, a maximum signal appears at theoutput of each multiplier/correlator 726, 728, 729. When the incomingsignal is thus on track, the delay lock loop 751 passes a clock signaland an acquisition signal to the mode control and acquisition device733. Any equivalent error generating device may be used to perform thefunction of the delay lock loop as would be known to persons of skill inthe art.

The delay element 752 also provides an on-time path that is used by thephase lock loop oscillator 750. The phase lock loop oscillator 750 iscentered on the correlation peak and provides the maximum carrier signalstrength. The data channel delay element 753 also places the datachannel to have the same alignment, on-time and maximum carrierstrength, as the phase lock loop path. The phase lock loop oscillator750 provides a coherent carrier reference to the coherent detector 758and to the delay lock loop 751. The orthogonal code set generator 740provides an orthogonal code, as assigned by the base station through theprocessor 732, to modulo-two adder 715, where the orthogonal code iscombined with the output of the noise-like code generator 741 to formthe local data spread spectrum reference signal. Since the base stationpilot code and the user data code channels are synchronized andtransmitted on the same RF carrier, the phase of the local code and thecarrier phase of the pilot channel, after acquisition, can be used todemodulate the user data channel. The reference signal coming from theadder 715 is delayed by delay element 753 and multiplied with theincoming received combined spread spectrum signal in themultiplier/correlator 725 to correlate out the user data channel. Theoutput of multiplier/correlator 725 is integrated in the bandpass filter755 to make the information channel arrive at the correlation peak fordetection by the coherent detector 758. The output of the coherentdetector 758 is integrated over the information bit period by theintegrate and dump circuit 760. The integrate and dump circuit 760samples the output at the time determined by the bit synchronizer 759.The bit synchronizer 759 is synchronized with the orthogonal code setgenerator 740 so that when the codes are synchronized the data bits arealso automatically synchronized. This occurs because the data in thebase station transmitter is also synchronized to the base stationorthogonal code generator. The output signal 775 is the user datamultiplexed with specific channel overhead data that is stripped out ofthe data signal by a demux, not shown, and sent to the processor 732.This overhead data includes power control messages, code phase alignmentmessages, mode change messages etc. These messages come into theprocessor over the processor input/output 771.

Orthogonal code set generator 740 is identical to orthogonal code setgenerator 702, and noise-like code generator 741 is identical tonoise-like code generator 703. Orthogonal code set generator 740 andnoise-like code generator 741 are clocked by mode control andacquisition device 733. Before acquisition, mode control and acquisitiondevice 733 uses a stable internal clock to provide timing to the codegenerators; after acquisition, the PLL oscillator 750 is slaved to theclock derived from the delay lock loop 751. The clock pulse generator730 is also slaved to the output of the mode control and acquisitiondevice 733.

As illustratively shown in FIG. 8 a and FIG. 8 b, a base stationaccording to the present invention includes a base station antenna 827,user data sources 800, pilot data source 801, orthogonal code setgenerators 802, 840, noise-like code generators 803, 841, eightmodulo-two adders, 810-815 and 817-818, a signal combiner 816, radiofrequency translator/modulator 820, a clock pulse generator 830, rangedelay device 834, processor 832, controller 836, code phase adjuster831, mode control and acquisition device 833, four bandpass filters 854,855, 856, 857, bit synchronizer 859, coherent detector 858, integrateand dump circuit 860, delay lock loop 851, delay elements 852, 853, fourmultiplier correlators 825, 826, 828, 829, phase lock loop oscillator850, power splitter 822, multicoupler 821, and carrier generator 819.FIG. 8 a and FIG, 8 b also shows processor input/output 871, user datainput 870, user data output 875, and radio frequency input/output 873.

FIG. 8 a and FIG. 8 b is illustrative of a base station that exhibitsthe features of this invention. There are many similarities between thebase station of FIG. 8 a and FIG. 8 b and the mobile terminal of FIG. 7a and FIG. 7 b. In the following discussion, the differences between thebase station and the mobile terminal are emphasized.

In FIG. 8 a and FIG. 8 b there are three data sources. In addition tothe user data and the pilot data as shown in FIG. 7 a and FIG. 7 b,there is a need for system data that is transmitted to all the usersthat are connected to the base station. This type of data includesgeneral system parameters, paging information, system synchronizationmarks, control information and channel assignments. Much of this systeminformation originates at the network central controller and is sent tothe base controller 836, over land lines, where it is adapted to theindividual cell. The processor 832 works in conjunction with thecontroller 836 to interface these messages into the base station. Thisis information that is generally broadcast so that all users can receiveit before they are assigned to a specific channel.

The system information that is transmitted to a specific user while themobile terminal is operating on an assigned channel is input to the userdata means at input 870 and is multiplexed with the user data. Thesystem data is also spread with a unique orthogonal code, generated bythe orthogonal code set generator 802, in adder 817 and is furtherrandomized by adding an additional noise-like PN code in adder 818. Thenoise-like PN code is generated by noise-like code generator 803. Therecan be several system data channels; each one spread with a uniqueorthogonal code, but all using the same noise-like PN code. The samenoise-like PN code is added to all the channels including all datachannels, all system channels and the pilot channel. There is only onepilot channel and it uses one of the unique orthogonal codes, usuallythe code that is all zeros. This means the noise-like PN code isessentially the pilot code, but it is also a component of all the othercodes. The concept of a pilot on the forward link is commonly acceptedand well documented in the prior art; see U.S. Pat. No. 5,228,056, U.S.Pat. No. 5,420,896, U.S. Pat. No. 5,103,459 and U.S. Pat. No. 5,416,797.There are also several means for generating different pilots fordifferent base stations, including deliberately introducing a fixed codephase shift; see U.S. Pat. No. 5,103,459 and U.S. Pat. No. 5,416,797.

FIG. 8 only shows one user data source 800, for illustrative purposes,but there will normally be many user data sources or channels, one foreach active user. Each active user will be assigned a unique orthogonalcode and will use the same noise-like PN code. Therefore, the input tocombiner 816 will normally include many user data channels, severalsystem channels and a pilot channel. The output of combiner 816 is acomposite spread spectrum signal that is modulated on the carrier,w_(c), in translator/modulator 820. The modulated composite spreadspectrum signal is sent to the base antenna 827 through multi-coupler821. The multicoupler 821 not only provides isolation between thetransmit and receive signals, as is done in the mobile terminal, but hasto also isolate multiple transmit signals from each other. An alternateapproach would be to combine the signals at a low power level and uselinear amplifiers for the final stages.

The clock pulse generator 830 is derived from a stable oscillator and isthe basic clock for the entire cell. Absolute time is maintainedthroughout the system. This same absolute time at all base stationsallows the mobile terminal to determine absolute time delay to severalbase stations, resulting in accurate geographical positiondetermination. The clock pulse generator 830 provides the clock for boththe orthogonal code generator 802 and the noise-like code generator 803.It also provides the clock for the orthogonal code generator 840 and thenoise-like code generator 841 when the reverse link is operating in theorthogonal code mode. When the receiver is not operating in theorthogonal code mode, and it has acquired an assigned user signal, theorthogonal code generator 840 and the noise-like code generator 841 usethe clock generated by the delay lock loop 851 as their clock source.

When the pilot receive channel has acquired the user pilot signal, onthe reverse channel, and the delay lock loop 851 is tracking theincoming pilot signal, the reference pilot code, produced by adding theoutputs of orthogonal code set generator 840 and the noise-like codegenerator 841 in adder 814, is in complete synchronization with thepilot signal from the user. When this state occurs, an output of adder814 is accepted by the range delay device 834 and the phase of thispilot code is compared to the phase of the base station pilot code,taken from the output of adder 813. With the assistance of the processor832, the range delay device 834 calculates the phase difference betweenthe two signals and places this value in memory in the processor 832.The value of the round trip delay is also sent to the mobile terminalthat is transmitting the user pilot signal, through input port 870 onuser data source 800, or as part of the set-up command on the assignmentchannel.

When the mobile terminal is in the pilot ranging orthogonal mode ofoperation, the base station is sending the user terminal ranginginformation and the user terminal is sending back user data on thereturn link in the orthogonal mode. There may be a small fixed offsetbetween the pilot channel range measurement and the correct phase toachieve maximum noise reduction on the orthogonal channel. To removethis offset, the processor 832 sends commands to the mobile terminal tomove the phase relationship between the user pilot and user data channelin fractions of a chip, one-eighth, one-tenth or one-sixteenth, whilethe processor 832 observes the output level of the integrate and dumpcircuit 860. When the peak output signal level is observed, that offsetis locked and maintained. This process calibrates the relationshipbetween the user pilot and user data channels. Once optimized, thisrelationship should not change significantly during the course of anormal transmission. It can always be re-instituted after a fixedinterval.

When the mobile terminal is in the mode of also transmitting anorthogonal pilot that is synchronized with the user data channel, thedelay lock loop 851 error voltage is sent to the processor 832, analyzedand supplemented with a predictive component, and transmitted to themobile terminal for use in correcting the phase of the composite signaltransmitted back by the mobile terminal. Since the error is detected inthe base station and the correction is made in the mobile station, thereis an inherent delay in the loop. This delay is small, however, incomparison to the normal movement of the user and, since the movement ofthe user will not normally change direction rapidly, a prediction can bemade based on the last measurements. If the path length has a suddenjump of several chips, then the mobile terminal is commanded to returnto the previous mode using the ranging information to reacquire. Thiswould only happen if a strong primary multipath was faded rapidly andthere was no existing secondary ray, but a new secondary ray appearedsoon after the loss of the first.

Therefore, according to this invention, the base station receiver canreceive data from the mobile terminal in one of four modes. The firstmode allows the mobile terminal to send an independent user pilot, notsynchronized with the base station, on the reverse link and the userdata channel is synchronized to this independent user pilot. The secondmode requires the user terminal to slave its user pilot to the pilot itreceives from the base station and the user data channel is synchronizedwith this slaved user pilot. This second mode allows the user terminalto receive round trip delay information for purpose of geolocation andrapid reacquisition. The third mode requires the user terminal to slaveits user pilot to the incoming base station pilot, as in the case ofmode two, but the user data channel operates in the orthogonal modeusing the ranging information received from the base station. The phaserelationship between the user pilot channel and the user data channel iscalibrated; one technique is described above, but there are many othertechniques that should be obvious to one skilled in the art. The userpilot carrier is also the carrier for the user data channel and can beused as the carrier reference for detecting the user data channel. Thefourth mode employs the slaved pilot implementation of mode three foracquisition but, after acquisition, phase shifts the user pilot code tobe synchronous with the user data channel, thus also making the pilot anorthogonal channel. This means the pilot no longer contributesinterference to the user data channels, within the cell, and can betransmitted at higher power levels.

The present invention may further comprise a spread-spectrum CDMAcellular radio communications method for communicating remote-messagedata from a mobile terminal to a base station over a duplex radiochannel. The method includes using a pilot on the return link to achieveorthogonality at the base station antenna.

The method comprises the steps of spread spectrum processing remotemessage data using a pseudo-noise code, generating a remote-pilotsignal, and combining the remote-pilot signal with thespread-spectrum-processed-remote-message data to generate a remote-CDMAsignal. The remote CDMA signal contains the remote-pilot signal and adata signal.

The method then comprises the steps of transmitting the remote-CDMAsignal from the mobile terminal to the base station on a reverse channelof the duplex radio channel. The base station receives the remote-CDMAsignal and splits the remote-CDMA signal into a pilot channel and a datachannel. The method then comprises the steps of generating a base-pilotsignal and generating a base-pilot-reference signal. Thebase-pilot-reference signal is split and delayed to generate an on-timeversion of the base-pilot-reference signal, an early version of thebase-pilot-reference signal, and a late version of thebase-pilot-reference signal. The on-time, the early and the lateversions of the base-pilot-reference signal are used to correlate out anon-time, an early, and a late version, respectively, of the remote-pilotsignal.

The method then comprises the steps of generating a base-data-referencesignal and correlating out the data signal using the base-data referencesignal. The phase of the remote-pilot signal is tracked and, responsiveto a peak in the remote-pilot signal, an acquisition signal is outputsignifying synchronization of the remote-pilot signal and thebase-pilot-reference signal. In response to the acquisition signal, thephase of the remote-pilot signal may be shifted to be synchronous withthe data signal. The remote-pilot signal may also be slaved to thebase-pilot signal.

The method then comprises the steps of measuring, in response to theacquisition signal, a code phase difference between the base-pilotsignal and the base-pilot-reference signal to determine the rangebetween the mobile terminal and the base station. The range istransmitted to the mobile terminal and, in response to the range, themobile terminal adjusts the phase of the pseudo-noise code to adjust anarrival time of the data signal at the base station and to achieveorthogonality at the base station.

It will be apparent to those skilled in the art that variousmodifications can be made to the spread-spectrum communications systemand method of the instant invention without departing from the scope orspirit of the invention, and it is intended that the present inventioncover modifications and variations of the spread-spectrum communicationssystem and method described herein provided they come within the scopeof the appended claims and their equivalents.

1. A method for geographically locating a mobile terminal within a wireless CDMA communication system having base stations with fixed locations, the method comprising: transmitting from a plurality of base stations a first spread spectrum signal having an associated code; receiving of the first spread spectrum signals transmitted by said plurality of base stations at the mobile terminal; for each received first spread spectrum signal, transmitting a second spread spectrum signal having an associated code time synchronized with that received first spread spectrum signal from the mobile terminal to said plurality of base stations, wherein the synchronizing of the associated code of the second spread spectrum signal with that received first spread spectrum signal is by despreading that received first spread spectrum signal using the associated code of the first spread spectrum signal, processing that despread received first spread spectrum signal by a delay lock loop, and adjusting a timing of the associated code of the first spread spectrum signal used for despreading and a clock pulse in response to the delay lock loop, and adjusting a timing of the associated code of the second spread spectrum signal in response to the adjusted timing of the clock pulse and the associated code of the first spread spectrum signal; receiving the second spread spectrum signals at the plurality of base stations; determining a delay between each base station and the mobile terminal based on in part a received timing of the second signals, wherein the determining a delay between each base station and the mobile terminal is by despreading that received second spread spectrum signal using the associated code of the second spread spectrum signal, processing that despread received second spread spectrum signal by a delay lock loop, and adjusting a timing of the associated code of the second spread spectrum signal used for despreading in response to the delay lock loop, and comparing a timing of the time adjusted associated code of the second spread spectrum signal and the associated code of the first spread spectrum signal; and determining the mobile terminal's geographic location based on in part round trip delay information between the mobile terminal and each base station of signals transmitted between the mobile terminal and the respective base stations.
 2. The method of claim 1 wherein the determining of the mobile terminal's geographic location is performed at the mobile terminal.
 3. The method of claim 1 wherein the base stations are time synchronized with each other.
 4. The method of claim 2 further comprising each base station transmits the determined delay between the mobile terminal and that base station.
 5. The method of claim 4 further comprising the mobile terminal receiving the transmitted determined delays.
 6. A mobile terminal for use in a wireless CDMA communication system having a plurality of base stations, each base station transmitting a first spread spectrum signal having an associated code, the mobile terminal comprising: means for receiving the first spread spectrum signals transmitted by said plurality of base stations at the mobile terminal; means for each received first spread spectrum signal, transmitting a second spread spectrum signal to said plurality of base stations having an associated code time synchronized with that received first spread spectrum signal, whereby enabling each base station to make a delay determination, wherein the synchronizing of the associated code of the second spread spectrum signal with that received first spread spectrum signal is by despreading that received first spread spectrum signal using the associated code of the first spread spectrum signal, processing that despread received first spread spectrum signal by a delay lock loop, and adjusting a timing of the associated code of the first spread spectrum signal used for despreading and a clock pulse in response to the delay lock loop, and adjusting a timing of the associated code of the second spread spectrum signal in response to the adjusted timing of the clock pulse and the associated code of the first spread spectrum signal; means for receiving the delay determination from each base station; and means for determining the mobile terminal's geographic location based on in part round trip delay information between the mobile terminal and each base station of signals transmitted between the mobile terminal and the respective base stations.
 7. The mobile terminal of claim 6 wherein the first and second spread spectrum signals are pilot signals.
 8. A wireless CDMA system for geographically locating a mobile terminal, the system comprising: a plurality of base stations with fixed locations, each base station comprising: means for transmitting a first spread spectrum signal having an associated code; means for receiving a second spread spectrum signal having an associated code transmitted by said plurality of base stations; means for determining a delay between the mobile terminal and that base station based on in part a received timing of the received second signal from the mobile terminal to said plurality of base stations; and means for transmitting the delay determination to the mobile terminal; and the mobile terminal comprising: means for receiving the first spread spectrum signals at the mobile terminal; means for each received first spread spectrum signal, transmitting the second spread spectrum signal having its associated code time synchronized with that received first spread spectrum signal, wherein the synchronizing of the associated code of the second spread spectrum signal with that received first spread spectrum signal is by despreading that received first spread spectrum signal using the associated code of the first spread spectrum signal, processing that despread received first spread spectrum signal by a delay lock loop, and adjusting a timing of the associated code of the first spread spectrum signal used for despreading and a clock pulse in response to the delay lock loop, and adjusting a timing of the associated code of the second spread spectrum signal in response to the adjusted timing of the clock pulse and the associated code of the first spread spectrum signal; means for receiving the delay determination from each base station; and means for determining the mobile terminal's geographic location based on in part round trip delay information of signals transmitted between the mobile terminal and the respective base stations.
 9. The system of claim 8 wherein the base stations are time synchronized with each other. 